The present invention relates to a method for producing a ferroelectric layer. The present invention relates in particular to a method for producing a ferroelectric layer for an integrated memory arrangement, and to a method for producing a storage capacitor.
An advantage of ferroelectric memory arrangements over conventional memory arrangements, such as DRAMs and SRAMs, is that the stored information is not lost, but remains stored, even in the event of an interruption in the voltage or current supply. For this purpose, storage capacitors are generally used for storing information, even in ferroelectric memory arrangements. In order to produce storage capacitors such as these, a ferroelectric material, for example SrBi2Ta2O9 (SBT) or Pb(Zr, Ti)O3 (PZT), is inserted between the electrodes of the capacitor. The nonvolatility of ferroelectric memory arrangements is based on the fact that, in the case of ferroelectric materials, the polarization which is applied by an external electric field is essentially retained even after the external electric field is switched off. In this case, the signal which can be read from the storage capacitors is higher when the level of polarization that can be applied to the ferroelectric material is higher. This means that, in order to make it possible to ensure a sufficiently high signal for reading from a storage capacitor, a high remanent polarization is required between the electrodes of the capacitor.
Ferroelectric materials are characterized in that they have microstructural domains which have an electrical polarization. The alignment of this polarization is linked to the orientation of the respective crystal lattice. Thus, for example in the case of PZT, the polarization is aligned in the direction of the crystallographic [001] axis. In the case of SBT, the vector of the electrical polarization is mainly parallel to the a axis ([100] orientation) or parallel to the b axis ([010] orientation). However, both axes are virtually equivalent, since this is a pseudotetragon lattice.
Since the crystals of ferroelectric materials are generally randomly oriented, the domains and hence the vectors of the electrical polarization are also oriented randomly. If an external electric field is now applied, then the polarization vectors of the individual domains are aligned such that they are as parallel as possible to the applied external field. For domains whose crystal is aligned such that the [100] axis is at right angles to the plates of the capacitor, this means that the entire polarization vector of the domains runs parallel to the external field. For differently oriented domains, only that component of the polarization vector which is parallel to the external field is relevant. The macroscopically measurable overall polarization, which is at right angles to the electrodes of the capacitor, is the sum of the individual polarizations of the domains. This sum becomes higher the more preferably the individual domains are aligned at right angles to the electrodes of the capacitor, that is to say for example in the case of SBT, the greater is the proportion of the crystal whose [100] axis is at right angles to the electrodes of the capacitor.
A high remanent polarization is of critical importance for the use of ferroelectric thin films in large scale integrated components, for example in integrated memory arrangements with structure sizes of less than 0.25 xcexcm since, in this case, not only is the surface area of the capacitor very small, but these structures also have a very large surface area in comparison to their volume. The surface area of ferroelectric materials is, however, always subject to damage, resulting from the structuring, and this leads to a reduction in the remanent polarization. In addition to the actual structuring of the ferroelectric materials, further processes, which are essential for the production of integrated components (for example forming gas heat treatment, TEOS oxide/SiO2 deposition etc) also lead to a degradation of the ferroelectric material and, in a corresponding manner, to a reduction in the remanent polarization. As a result of this process-dependent reduction in the polarization, it is important to have a material whose polarization is as high as possible before the start of the structuring and the subsequent processes.
Accordingly, it is preferable to monitor the orientation of the ferroelectric material, that is to say for example in the case of SBT to produce as much [100] or [010] oriented material as possible, or in the case of PZT to produce as much [001] oriented material as possible, whose polarization vectors are at right angles to the electrodes of the storage capacitor. In the case of SBT, other orientations are also useful within the a, b plane (for example [110]). Although these have less polarization, the polarization is, however, still considerably greater than if there were a large number of domains aligned along the c axis. If, in a corresponding manner, it were possible to give ferroelectric materials a preferred orientation, this would result in a very high remanent polarization.
The object of the present invention is to specify a method for producing a ferroelectric layer, which is able to align the majority of the domains of the layer along a predetermined direction, and to specify a method for producing storage capacitors which have a high remanent polarization.
According to the invention, this object is achieved by a method for producing a ferroelectric layer and by a method for producing a ferroelectric storage capacitor. Furthermore, a ferroelectric layer which is produced in this way and a memory arrangement which is produced in this way are provided. Further advantageous embodiments, characteristics and aspects of the present invention can be found in the claims, in the description and in the attached drawings.
According to the invention, a method is provided for producing a ferroelectric layer, which method has the following steps:
a) a substrate is provided,
b) the material for the subsequent ferroelectric layer is applied to the substrate,
c) heat treatment is carried out in the presence of an electric field which is aligned along a predetermined direction, so that the material is changed to a ferroelectric phase.
The method according to the invention has the advantage that the application of an external electric field simplifies the crystallization of the material on the basis of the predetermined direction. Without wishing to impose any restriction, the inventors are of the opinion that this can be explained by the fact that the crystallization will always run in the direction which results in a state with the least Gibb""s free energy. Normally, this will be a material with randomly oriented crystals, since this allows a state with very high entropy to be produced. However, if an external electric field is applied, then an additional energy term is added, which describes the interaction between the external field and the ferroelectric polarization. This energy term is generally a minimum when the polarization of the resultant ferroelectric material runs parallel to the external field. This also means that the stronger the external field, the more strongly the material is aligned in the predetermined direction. A ferroelectric material can accordingly be produced in which the crystallization, for example in the case of SBT, is aligned in a preferred manner in the [100], [010] or [110] direction or, in the case of PZT in the [001] direction.
Suitable choice of the alignment of the field which is applied for crystallization allows ferroelectric layers to be produced whose domains are aligned in a preferred manner such that their polarization vectors are at right angles to the electrodes of the storage capacitor in a memory cell. This means that, during operation of the memory arrangement, the entire polarization vector of the domains runs essentially parallel to the field of the storage capacitor, and a correspondingly high remanent polarization is produced.
According to one preferred embodiment, the ferroelectric layer is a strontium bismuth titanate layer (SBT, SrBi2Ta2O9) and heat treatment is carried out at a temperature range of between 500xc2x0 and 820xc2x0 C., preferably at a range of between 700xc2x0 and 800xc2x0 C. It is particularly preferable for the heat treatment to be carried out at a temperature range of between 700xc2x0 and 750xc2x0 C. According to a further preferred embodiment, the ferroelectric layer is a lead zirconate titanate layer (PZT, Pb(Zr, Ti)O3), and the heat treatment is carried out at a temperature range of between 400xc2x0 and 600xc2x0 C. The heat treatment is preferably carried out over a time period of 5 to 90 minutes, preferably a period of 10 to 30 minutes.
It is furthermore preferable for the field strength of the electric field to be in a range between 1 and 100 kV/cm, preferably a range between 20 and 40 kV/cm.
According to one preferred embodiment, the substrate is used as an electrode for application of the electric field. In this case, it is particularly preferable for a noble metal electrode, in particular a platinum electrode, to be provided on the surface of the substrate. According to one preferred embodiment, a conductive plate is used as the second electrode, and is arranged above the material of the subsequent ferroelectric layer. According to a further preferred embodiment, a plasma is used as the second electrode, and is produced above the material of the subsequent ferroelectric layer. The use of a plasma above the subsequent ferroelectric layer has the advantage that the plasma extends directly as far as the material, so that the applied electric field can be applied directly to the material. In this case, it is preferable for the plasma to be produced either by an alternating frequency or voltage pulses. In this case, it is particularly preferable for the substrate to be immersed in the plasma by means of voltage pulses. When an additional conductive plate is used, a gap is generally provided between the plate and the material, in which there is likewise an electric field, which means that a higher voltage needs to be applied in order to form the electric field.
According to one preferred embodiment, the heat treatment is carried out either in an N2/O2 atmosphere or in an He/O2 atmosphere. It is furthermore preferable for the heat treatment to be carried out at a pressure range of between 0.05 and 10 Pa.
According to a further preferred embodiment, the material of the ferroelectric layer is applied to the substrate by means of a CVD method. In this case, it is particularly preferable for the material of the subsequent ferroelectric layer to be applied to the substrate as an essentially amorphous film.
The invention will be explained in more detail in the following text with reference to the drawings.